Surfactants based on glycitan esters of fatty acids, most notably sorbitan esters of fatty acids, are gaining increasing attention due to their advantages over other surfactants with regard to their notable dermatological properties and good compatibility with standard products, as well as their favorable environmental profile. In addition, their low toxicity, good biocompatibility, and fast biodegradation make this class of molecules very attractive, not only for personal care products but also for a large range of technical applications.
More particularly, glycitan esters of unsaturated fatty acids are nonionic, amphiphilic materials that provide emulsifying, and wetting properties that are comparable to those of other nonionic surfactants. They are conventionally produced by reacting intramolecular condensates of glycitols having four or more carbons with a fatty acid. The fatty acids used to make such fatty acid esters are prepared by hydrolysis of triglycerides, which typically originate from animal or vegetable fats. Consequently, the fatty portion of the acid will typically have 6-22 carbons with a mixture of saturated and internally unsaturated chains. Depending on the source, the fatty acids often have a preponderance of C16 to C22 components. For instance, hydrolysis of soybean oil provides saturated palmitic (C16) and stearic (C18) acids and unsaturated oleic (C18 mono-unsaturated), linoleic (C18 di-unsaturated), and α-linolenic (C18 tri-unsaturated) acids. The unsaturation in these acids has either exclusively or predominantly a cis-configuration. Thus, traditional sources of fatty acids used to produce saturated and unsaturated glycitan fatty esters generally have predominantly or exclusively cis-isomers and lack relatively short-chain (for example, C10 or C12) unsaturated fatty portions.
Improvements in metathesis catalysts (see J. C. Mol, Green Chem. 4 (2002) 5) provide an opportunity to generate reduced chain length, monounsaturated feedstocks, which are valuable for making detergents and surfactants, from C16 to C22-rich natural oils such as soybean oil or palm oil. Soybean oil and palm oil can be more economical than, for example, coconut oil, which is a traditional starting material for making detergents. As Professor Mol explains, metathesis relies on conversion of olefins into new products by rupture and reformation of carbon-carbon double bonds mediated by transition metal carbene complexes. Self-metathesis of an unsaturated fatty ester can provide an equilibrium mixture of starting material, an internally unsaturated hydrocarbon, and an unsaturated diester. For instance, methyl oleate (methyl cis-9-octadecenoate) is partially converted to 9-octadecene and dimethyl 9-octadecene-1,18-dioate, with both products consisting predominantly of the trans-isomer. Metathesis effectively isomerizes the cis-double bond of methyl oleate to give an equilibrium mixture of cis- and trans-isomers in both the “unconverted” starting material and the metathesis products, with the trans-isomers predominating.
Cross-metathesis of unsaturated fatty esters with olefins generates new olefins and new unsaturated esters that can have reduced chain lengths and that may be difficult to make otherwise. For instance, cross-metathesis of methyl oleate and 3-hexene provides 3-dodecene and methyl 9-dodecenoate (see also U.S. Pat. No. 4,545,941). Terminal olefins are particularly desirable synthetic targets, and Elevance Renewable Sciences, Inc. recently described an improved way to prepare them by cross-metathesis of an internal olefin and an α-olefin in the presence of a ruthenium alkylidene catalyst (see U.S. Pat. Appl. Publ. No. 2010/0145086). A variety of cross-metathesis reactions involving an α-olefin and an unsaturated fatty ester (as the internal olefin source) are described. Thus, for example, reaction of soybean oil with propylene followed by hydrolysis gives, among other things, 1-decene, 2-undecene, 9-decenoic acid, and 9-undecenoic acid.